Volatilisation and catalytic effects of alkali and alkaline earth metallic species during the pyrolysis and gasi®cation of Victorian brown coal. Part III. The importance of the interactions between volatiles and char at high temperature q Hongwei Wu, Dimple M. Quyn, Chun-Zhu Li * CRC for Clean Power from Lignite, Department of Chemical Engineering, P.O. Box 36, Monash University, Victoria 3800, Australia Received 20 May 2001; accepted 13 December 2001; available online 28 January 2002 Abstract Anoveltwo-stage¯uidised-bed/®xed-bedreactorwasdesignedtoinvestigatetheeffectsofvolatile-charinteractionsonthevolatilisation of alkali and alkaline earth metallic AAEM) species during the pyrolysis of Victorian brown coal at 900 8C. With the two-stage reactor con®guration,theAAEM-freevolatilesgeneratedfromthepyrolysisoftheH-formcoalinthe¯uidisedbedcameintodirectcontactwiththe char from NaCl-loaded or Na-form coals in the ®xed bed. The results indicated that the interactions between the volatiles, especially free radicals in the volatiles, and the char particles enhanced the volatilisation of Na from the char drastically. However, such radical±char interactions resulted in little volatilisation of Mg and Ca, indicating the importance of valence of the AAEM species. The degree of the volatile±char interactions was also related to the ageing of the char and the chemical form of AAEM species in the coal substrate. The volatilesinteractedmorestronglywiththenascentcharthantheagedchar,indicatingthattheAAEMspeciesexistedintheagedcharinmore stable forms than in the nascent char. q 2002 Elsevier Science Ltd. All rights reserved. Keywords: Sodium; Volatilisation; Volatiles; Char; Radical; Brown coal 1. Introduction ThealkaliandalkalineearthmetallicAAEM)speciesin solidfuels,eitherpresentedinherently,orloadedarti®cially, areoneoftheimportantconsiderationsfortheutilisationof these fuels [1±5]. Volatilisation of AAEM species was observed during the pyrolysis of biomass [6±8] and coal [9±11] and strongly depended on temperature. The gas atmosphere surrounding the coal particles was also found to in¯uence the volatilisation of AAEM species during the pyrolysis of a Victorian brown coal in a recent study [12]. However, the fundamental mechanisms leading to the volatilisation of AAEM species during pyrolysis are still poorly understood. Although there was a lot of work done on the volatilisation of the AAEM species during the gasi- ®cation of graphite with loaded AAEM salts [13±18], the obtained mechanisms in those studies are unlikely to be directly applicable to coal pyrolysis because signi®cant interactions among the AAEM species, volatiles and char may occur during coal pyrolysis. The purpose of our study is to investigate the volatilisa- tion of AAEM species during the pyrolysis of Victorian brown coals, taking into consideration the different forms of the AAEM species in the coal substrates. Part I [19] of this series revealed that NaCl in the coal substrates was volatilised mainly as Na and Cl separately rather than as NaCl.PartII[20]oftheseriesstudiedtheeffectsofchemi- calformandvalenceonthevolatilisationofAAEMspecies. The fundamental mechanisms at temperatures less than 600 8C were also proposed. The data in the ®rst two parts implied that the interactions between volatiles and char might play signi®cant roles in the volatilisation of AAEM species at high temperatures. The purpose of this study, which is Part III of the series, is to investigate the fundamental mechanisms governing the volatilisation of AAEM species during the pyrolysis of brown coals at high temperatures i.e. 900 8C).Anovelreactorisdesigned to obtain direct experimental evidence on the effects of volatile±char interactions on the volatilisation of AAEM species. Fuel812002)1033±1039 0016-2361/02/$ - see front matter q 2002 Elsevier Science Ltd. All rights reserved. PII:S0016-236102)00011-X www.fuel®rst.com * Correspondingauthor.Tel.: 161-3-9905-9623;fax: 161-3-9905-5686. E-mail address: chun-zhu.li@eng.monash.edu.au C.-Z. Li). q Published ®rst on the web via Fuel®rst.comÐhttp://www.fuel®rst.com